Van life · 11 min · updated 2026-05-30
Working out a van's power budget on the back of an envelope
How to add up what you actually draw in a day, and what that means for panels and battery — in amp-hours, without the marketing.
Count the day, not the peak
Every device has a draw in amps and a number of hours you actually use it. Multiply them, add them up, and you have your daily amp-hour figure. A 12V lamp at 0.4 A for five hours is 2 Ah. A fridge cycling at an average of 1.2 A for twenty-four hours is roughly 29 Ah, and it will dominate your total. Most first budgets are wrong because they count the laptop and forget the fridge.
Then halve your solar optimism
A 100 W panel does not give you 100 W. Between angle, temperature, cloud, and controller losses, a reasonable planning figure in good conditions is four to five useful hours a day, and in winter or under trees it can be one. Size for the bad week, not the brochure day, or plan an alternative charging route for it.
Give yourself a reserve you never touch
Deep-cycling a battery to empty shortens its life sharply. Plan to use roughly half of nominal capacity and treat the rest as untouchable. A 100 Ah battery is a 50 Ah battery in practice, and that number is what has to cover your daily total plus one bad day.